A fixing apparatus used in an electrophotographic image forming apparatus such as a copying machine, a printer, and the like, is typically constituted by a pair of rollers (fixing roller and pressure roller) which are pressed against each other. As such a fixing apparatus, a heat roller type fixing apparatus is frequently used, wherein halogen heaters disposed in both the rollers or a halogen heater disposed in one of the rollers heat the pair of rollers at a predetermined temperature (fixing temperature), and a recording paper P on which an unfixed toner image is formed is allowed to pass through a pressing section (fixing nip area) of the pair of rollers so as to fix the toner image under heat and pressure.
Especially, in a fixing apparatus provided in a color image forming apparatus, it is general to use an elastic roller having an elastic layer which is made of silicon rubber or the like and which is provided on a fixing roller surface layer.
The elastic roller is used as the fixing roller, so that the fixing roller surface is elastically deformed corresponding to an uneven surface of the unfixed toner image and is in contact with the toner image so as to cover the toner image. This allows the color unfixed toner image whose toner amount is larger than that of monochrome unfixed toner image to be favorably fixed under heat. Further, due to strain release of the elastic layer which occurs in the fixing nip area N1, it is possible to improve a releasing property with respect to color toner which is more likely to offset than monochrome toner. Further, a nip shape of the fixing nip area N1 has a concave upward (that is, the nip shape is a so-called inverse nip shape), so that it is possible to more favorably separate the recording paper P from the fixing roller. As a result, it is possible to separate the recording paper P without using any separation means such as a separation claw (self stripping), so that it is possible to prevent insufficient image formation which is caused by the separation means.
However, in the fixing roller having the elastic layer, the elastic layer cannot sufficiently conduct heat. Thus, in case where the heating means is provided in the fixing roller, heat is less efficiently conducted, so that it takes longer time to warm up. In case where the process is carried out at higher speed, the fixing roller cannot follow the process.
As a method for solving these problems, an arrangement in which external heating means is brought into contact with the fixing roller surface so that the fixing roller is heated from the outside (external heat fixing process) is known. Especially, an arrangement in which an endless belt is used as the external heating device (external belt heat fixing process) has been recently proposed (Patent document 1: Japanese Unexamined Patent Publication No. 198659/2004 (Tokukai 2004-198659; published on Jul. 15, 2004) and Patent document 2: Japanese Unexamined Patent Publication No. 189427/2005 (Tokukai 2005-189427; published on Jul. 14, 2005).
However, the above external belt heat fixing process that was conventionally carried out offers a high ability to supply heat to the fixing roller. In addition, heat is transmitted from the heat source to the fixing roller surface, passing though a plurality of members (an endless belt and support rollers for suspending the endless belt) provided between the heat source and the fixing roller surface. As a result, wide temperature variations occur between the heat source and the fixing roller surface. That is, temperature variations between the heat source, the support rollers, the endless belt, and the fixing roller surface are as follows: heat source>support rollers>endless belt>fixing roller surface.
In a situation where rotation of the fixing roller is stopped, a temperature of each of the support rollers is raised by heat of the heat source even if heat supply from the heat source is stopped. Furthermore, an area of the endless belt where the endless belt is in contact with the support roller is small in heat capacity and thus instantly reaches a temperature that is the same as a temperature of the support rollers. Accordingly, a temperature of the endless belt becomes higher than a temperature that the endless belt has during the fixing operation. This can cause heat damage to the endless belt.
Also, there occurs rise in temperature of the fixing roller surface in an area where the fixing roller is in contact with the support rollers via the endless belt. This causes heat damage to the fixing roller surface or temperature variations in the fixing roller surface. Under such a situation, if the fixing operation is restarted, there occurs unevenness of the image.
That is, as compared with the external heating process in which rollers are brought into contact with a fixing member, a heating nip area becomes wider in the external belt heat fixing process in which the endless belt is brought into contact with the fixing member. Therefore, even if the endless belt has a smaller heat capacity, the endless belt can supply a large amount of heat to the surface of the fixing member, and a temperature of the fixing member can excellently respond to a high-speed fixing. However, when the operation of the fixing member is stopped after the end of the fixing, a temperature of the endless belt in an area where the endless belt is set over the support rollers rapidly increases, the endless belt and the fixing member are partly heated and therefore deteriorated (hereinafter such phenomenon is referred to as overshoot).